The present invention relates to the control of certain parasitic weeds and to weed control compositions suitable for this purpose, as well as to certain new chemical compounds possessing the requisite biological activity. More particularly the invention relates to the control of the weeds Striga hermonthica, S. asiatica (lutea), Orobanche crenata, O. ramosa and O. aegyptiaca which are parasitic on certain economically important crops such as sorghum, maize, sugar cane and/or broad beans.
Striga causes serious losses in sorghum production and its control is highly desirable since sorghum is the principal subsistence cereal grain for more than 300 million people living in arid tropcial countries. One of the reasons it is difficult to control this weed is that its seeds can remain viable in the soil for as long as 20 years, only germinating in close proximity to the host plant. Thus, control of Striga by rotation of crops or leaving the sorghum fields fallow for a year or more is ineffective since the seeds germinate only when the host plant is sown and starts to grow. Control by planting "false hosts", i.e. plants which trigger germination of the parasite but which do not act as hosts, is usually uneconomical since such planting must replace the desired crop. Another reason Striga is difficult to control is that since it is parasitic in nature, it draws all its nourishment from its host plant, e.g. sorghum, and is not readily amenable to control by normal weed killers, even those which are selective against certain classes of the weeds.